The same Hong Kong scientists who followed SARS from the moment it emerged as a mystery disease until they had identified its cause warned on Monday that swine flu poses an even greater challenge. While scientists have studied influenza for many years, the nature of the disease makes it a tough enemy to combat. With Severe Acute Respiratory Syndrome, or SARS, patients developed symptoms around the same time they became contagious. But with the flu, a person can spread the infection days before they feel sick enough to go to a doctor. "The flu is a known devil," says Malik Peiris, one of the scientists at Hong Kong University who helped trace the 2003 outbreak of SARS to the civet cat. "This is a different ballgame."

It's no wonder that Hong Kong has taken some of the toughest measures of any country in the effort to prevent and control the spread of the H1N1 Influenza A swine flu, which has killed more than 100 people in Mexico and infected several in the U.S., Canada and Spain, with suspected cases in Israel and New Zealand. Surgical masks, quarantines and empty streets are all too familiar for the city's 7 million residents, who saw their normally bustling lives screech to a halt six years ago, when SARS killed nearly 300 people. (See pictures of the swine flu outbreak in Mexico.)

"If there is anywhere in the world that took a beating by SARS, it was Hong Kong," says Peter Cordingley, spokesman for the World Health Organization (WHO) in Manila. "The lesson was learned." Drawing on the past, Hong Kong has already issued travel advisories and stepped up controls at airports, including the use of infrared temperature scans and the detainment of travelers arriving with flu-like symptoms. (Read "The Truth About SARS.")

The Hong Kong government now reacts to public health threats like a well-oiled machine. Although authorities have identified just three people with flu-like symptoms who have traveled or have been exposed to someone who has traveled in recent days, they are on high alert. Two of the three people with respiratory ailments, a 77-year-old woman and her 4-year-old granddaughter, tested negative for H1N1. The third, a 27-year-old woman who had been to San Francisco, tested positive for a human influenza subtype, not swine flu. Rated "serious" on the government's influenza-alert scale, swine flu was named a notifiable disease on Monday, which means doctors are required by law to report any suspected cases.

It's a speedy reaction from a government that was criticized for not doing enough to curb the spread of SARS, which led to the resignation of the acting health chief, Yeoh Eng-kiong. "While we tragically suffered the 2003 SARS outbreak, it gave us a lot of valuable insight and practical experience in managing a large-scale outbreak," said Gabriel Matthew Leung, Undersecretary for Food and Health, at a news conference in Hong Kong on Monday afternoon. "It certainly prepared us very well for what may come."

After some initial missteps that allowed SARS to spread at an almost uncontrollable rate, Hong Kong was eventually heralded for its accomplishments in helping to rid the world of SARS and, a few years later, for its quick response to multiple avian flu scares. "I always think back to during the bird flu  some wild bird drops out of the sky and is found in Hong Kong. It finishes up in a lab, being dissected," says WHO's Cordingley. "Anywhere else it would be chucked in the garbage." (See pictures of change in Hong Kong.)

Those same successes  and even the mistakes  may now provide valuable lessons for the global community. "One of the things that let Hong Kong down during SARS was poor infection control in hospitals," Cordingley says. Transmission of the disease proved particularly troublesome at Hong Kong's Prince of Wales Hospital, where one "superspreader" patient infected more than 90 people, including many health workers. "At that time, the number of isolation beds and isolation wards was very limited, so we really didn't have the infrastructural capacity to deal with such a major infectious-disease outbreak," Hong Kong University's Peiris says. Now the situation has greatly improved, he adds, with infection-control policies that minimize unnecessary movement of people in and out of hospitals. Currently, the government has 1,400 beds available for infectious-disease patients and 20 million doses of antiflu medication  including Tamiflu and Relenza  according to Food and Health Undersecretary Leung.

WHO's Cordingley also cautions against underreporting the extent of the spread of the disease, hinting at what happened with China's Ministry of Health, which came under fire for covering up much of the deadly SARS outbreak. "If you have got a problem, you have to speak up," he says. So far WHO has been pleased with communication from Mexico and elsewhere, he says, adding that reporting has been "instantaneous." (See pictures of the return of bird flu.)

But the most important lessons we can learn from the SARS outbreak, says Peiris, may be the simplest ones, taught by mothers the world over: wash your hands, cover your mouth when you cough and avoid touching your eyes, nose and mouth. The message seems to have sunk in in Hong Kong, where even on days without deadly flu threats many people wear face masks and hand-sanitizer dispensers are just about everywhere. There's no question that Hong Kong is as prepared as it can be to tackle swine flu, but for now, just like the rest of the world, it waits. "We have to wait and see whether the virus gets to us," Peiris says. "I suspect it will, but I don't know how soon."